Re: [eclipse-clp-users] Backtracking over current predicate only

From: Kish Shen <kisshen_at_...5...>
Date: Fri, 14 May 2010 16:57:29 +0100
Christian Wirth wrote:
> Hi,
> 
> how can i define a cut that allows backtracking over the current 
> predicate but not over another predicate ?
> example:
> 
> pred(aPred(X)) :- call(X).
> pred(X) :- anotherCall(X).
> 
> i now want to define a cut that creates all solutions for call(X), if 
> the first perdicate gets matched, but it should prevent matching the 
> other one.
> I'm currently using pred(X) :- 
> functor(X,F,A),not(F==aPred),anotherCall(X), but i want to replace this 
> because speedup is very essential.
> 
> .. hope this is possible.
> 

Hi Christian,

If by 'creates all solutions for call(X)' you mean 'allows the 
backtracking over call(X)' as you implied in your first paragraph, then 
what you want is referred to as the `soft cut' in Prolog literature, and 
in ECLiPSe, this is provided by the *->/2 construct (see the reference 
manual entry for *->/2 (you can use help (*->)/2 at the ECLiPSe 
top-level to access this information).

Using soft cut, your example can be written as:

pred(X0) :-
    X0 = aPred(X), 	
    ( call(X) *-> true ; anotherCall(X0))

(I have duplicated your aPred(X) wrapper behaviour -- did you truly 
intend this behaviour?)

anotherCall(X0) is only executed if call(X) has no solution at all; 
otherwise all solutions of call(X) can be obtained via backtracking.

A warning: support for soft cuts is quite controversial within the 
Prolog community, and my suggestion is that you should avoid using it if 
possible.

Cheers,

Kish


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Received on Fri May 14 2010 - 15:57:38 CEST

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